In a conventional vertical MOSFET (metal oxide semiconductor field effect transistor) device, it is desirable to minimize the drain-to-source resistance or RDS(on) of the device. RDS(on) is proportional to the amount of power consumed while the MOSFET device is on so that reducing RDS(on) reduces the amount of power consumed by the MOSFET device. RDS(on) could be reduced by increasing the dopant (or carrier) concentration in the drift region of the device. However, it may not be desirable to increase the dopant concentration, because increasing the dopant concentration reduces the breakdown voltage of the device. Conversely, the carrier concentration in the drift region of the MOSFET device cannot be reduced to increase the breakdown voltage without also undesirably increasing RDS(on).
U.S. Pat. No. 5,216,275 describes semiconductor devices with increased breakdown voltages and improved on-resistance properties. The devices of the type described in this patent are referred to as “superjunction” devices. Each of the described superjunction devices comprises a composite buffer layer. The composite buffer layer has alternating doped P and N regions that are charge balanced. According to the scientific literature, superjunction transistor devices exhibit 5-100 times lower specific on-resistance (Ron,sp) than conventional high voltage MOSFET devices.
While such superjunction transistor devices exhibit high breakdown voltages and low on-resistance, they are difficult to manufacture. For a superjunction device to function properly, the alternating P and N doped regions in the composite buffer layer must be doped with the same amount of charge material to achieve a perfect charge balance. This is difficult to achieve in practice. See, for example, Shenoy et al., “Analysis of the Effect of Charge Imbalance on the Static and Dynamic Characteristics of the Super Junction MOSFET”, Proc. of the ISPSD '99, pp. 95-98, 1999. In addition, because it is extremely difficult to precisely balance the doping in the composite buffer layer of a superjunction transistor device, the practical maximum electrical field achievable in the composite buffer layer is limited to approximately 2×105 V/cm. The practical maximum electrical field achieved by a superjunction transistor device limits its breakdown voltage.
It would be desirable to provide for an improved semiconductor device that is less difficult to manufacture and that has a higher breakdown voltage and a lower on-resistance than the superjunction devices described above.